Sheet conveying apparatus and image forming apparatus

ABSTRACT

A sheet conveying apparatus includes a detecting unit configured to detect a sheet; a conveying unit including a driving roller and a driven roller that rotates in conformity with the driving roller and configured to convey the sheet by holding the sheet between the driving roller and the driven roller; a disk configured to rotate in correspondence with the driven roller and including multiple slits arranged in a periphery thereof; first and second sensors configured to detect the multiple slits and output detection signals, the first and second sensors being provided substantially opposite to each other where a rotational axle of the disk is positioned at a center between the first and second sensors; and a conveyance distance calculating unit configured to calculate a sheet conveying distance based on a detection result of the detecting unit and the detection signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet conveying apparatus and an image forming apparatus.

2. Description of the Related Art

In the field of commercial printing, the style of printing diverse variable small lot data is shifting from the conventional style of using an offset printing machine to a POD (Print On Demand) style using an electrophotographic type image forming apparatus. For example, the electrophotographic type image forming apparatus is desired to provide performance (e.g., registering accuracy, image uniformity) that is equivalent to that of an offset printing machine.

The causes of registration deviation by the image forming apparatus can be categorized into the error of registration in a vertical/horizontal direction, the error of skew between a recording medium and a printed image, and the expansion/shrinkage of an image when transferring a toner image. Further, in a case of using an image forming apparatus including a fixing unit, the heating of a recording medium by the fixing unit may cause the recording medium to shrink. The shrinkage of the recording medium may cause an error of image magnification and lead to registration deviation.

The registration deviation can be prevented by, for example, measuring the lengths of a sheet in the sheet-conveying direction before and after a toner image is fixed thereto, calculating an expansion/contraction ratio (shrinkage rate) of a sheet, and correcting an image to be printed on a back side of a sheet based on the calculated shrinkage rate.

Accordingly, there is proposed a sheet-length measuring apparatus including a rotary encoder and a length measurement roller that contacts a conveyed sheet and rotates in conformity with the conveyed sheet (see, for example, Japanese Laid-Open Patent Publication Nos. 2011-6202 and 2011-20842). The sheet-length measuring apparatus measures a length of a sheet in a sheet-conveying direction based on a rotation amount of the length measurement roller detected by the rotary encoder.

However, with the sheet-length measuring apparatuses proposed in Japanese Laid-Open Patent Publication Nos. 2011-6202 and 2011-20842, the sheet-measurement results may be inconsistent in a case where an encoder disk of the rotary encoder is eccentric with respect to the length measurement roll.

For example, FIG. 9 is a graph illustrating the number of pulses (pulse count) output from an encoder sensor provided in an encoder disk during constant-speed rotation of the length measurement roller in a case where the encoder disk is eccentric with respect to an axis of the length measurement roller.

As illustrated in FIG. 9, even in a case where the length measurement roller is rotated at a constant speed, the cycle of pulses output from the encoder sensor may change when the encoder disk is eccentric with respect to the axis of the length measurement roller. Thus, the number of pulses may cyclically change. Accordingly, even if a time interval T of a period t1-t2 (period from time t1 to time t2) and a time interval of a period t3-t4 (period of time t3 and t4 are the same, the number of pulses during these two periods vary, that is, the number of pulses during period t1-t2 is n1 whereas the number of pulses during period t3-t4 is n2. Therefore, depending on the timing of measuring sheet length, the measurement results may become inconsistent. As a result, the accuracy of measuring sheet length is degraded.

SUMMARY OF THE INVENTION

The present invention may provide a sheet conveying apparatus and an image forming apparatus that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.

Features and advantages of the present invention are set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a sheet conveying apparatus and an image forming apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.

To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an embodiment of the present invention provides a sheet conveying apparatus including a detecting unit configured to detect a sheet; a conveying unit including a driving roller and a driven roller that rotates in conformity with the driving roller and configured to convey the sheet by holding the sheet between the driving roller and the driven roller; a disk configured to rotate in correspondence with the driven roller and including multiple slits arranged in a periphery thereof; first and second sensors configured to detect the multiple slits and output detection signals, the first and second sensors being provided substantially opposite to each other where a rotational axle of the disk is positioned at a center between the first and second sensors; and a conveyance distance calculating unit configured to calculate a sheet conveying distance based on a detection result of the detecting unit and the detection signals.

Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating a configuration of a portion of an image forming apparatus according to an embodiment of the present invention;

FIG. 3 is a schematic diagram illustrating a sheet conveying apparatus 100 according to an embodiment of the present invention;

FIG. 4 is a plan view of a sheet conveying apparatus according to an embodiment of the present invention;

FIG. 5 is a schematic diagram illustrating the number of pulse counts of pulse signals output from sensors of a rotary encoder during constant rotation of a driven roller according to an embodiment of the present invention;

FIG. 6 is a schematic diagram illustrating signal outputs from corresponding sensors according to an embodiment of the present invention;

FIG. 7 is a block diagram illustrating a functional configuration of an image forming apparatus according to an embodiment of the present invention;

FIG. 8 is a schematic diagram illustrating outputs of a start trigger sensor, a stop trigger sensor, and sensors according to an embodiment of the present invention; and

FIG. 9 is a graph illustrating the number of pulses (pulse count) output from an encoder sensor provided in an encoder disk during constant-speed rotation of a length measurement roller in a case where the encoder disk is eccentric with respect to an axis of the length measurement roller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, embodiments of the present invention are described with reference to the accompanying drawings. Throughout the drawings, like components/parts are denoted with like reference numerals as those of the first embodiment and are not further explained.

<Configuration of Image Forming Apparatus>

FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus 101 according to an embodiment of the present invention.

The image forming apparatus 101 includes an image forming unit constituted by, for example, a tandem image forming apparatus 54, an intermediate transfer belt 15, a secondary transfer apparatus 77, and a fixing apparatus 50. The image forming apparatus 101 forms an image on a sheet S of a recording medium (e.g., sheet of paper, OHP (Over-Head Projector) sheet).

The tandem image forming apparatus 54 includes multiple developing devices 53 y, 53 m, 53 c, and 53 k arranged along the intermediate transfer belt 15. For the sake of convenience, the developing devices 53 y, 53 m, 53 c, and 53 k may also be hereinafter simply referred to as developing devices 53 by omitting the letters “y, m, c, k” from reference numerals 53 y, 53 m, 53 c, and 53 k. An exposing apparatus 55 is provided above the tandem image forming apparatus 54. Each of the developing devices 53 includes a corresponding photoconductor drum 71 (71 y, 71 m, 71 c, 71 k) serving as an image carrier for carrying a toner image of a corresponding color.

Further, a primary transfer roller 81 (81, 81 m, 81 c, 81 k) is provided in a primary transfer area at which a toner image is transferred from a corresponding photoconductor drum 71 to the intermediate transfer belt 15. The primary transfer roller 81 is provided in a manner facing the photoconductor drum 71 and having the intermediate transfer belt 15 interposed (sandwiched) therebetween.

The secondary transfer apparatus 77 is provided on a side opposite from the tandem image forming apparatus 54 interposed by the intermediate transfer belt 15. The secondary transfer apparatus 77 is positioned on a downstream side of a sheet conveying direction of the intermediate transfer belt 15. The secondary transfer apparatus 77 transfers an image on the intermediate transfer belt 15 by pressing a secondary transfer roller 14 against a roller (opposite secondary transfer roller) 62 that faces the secondary transfer roller 14 and applying a transfer electric field to the roller 62. The secondary transfer apparatus 77 changes a parameter of a transfer condition (e.g., transfer current of the secondary transfer roller 14) in accordance with, for example, the type of the sheet S.

Further, the image forming apparatus 101 also includes a sheet conveying apparatus 100 that can calculate, for example, the distance in which the sheet S is conveyed (sheet conveying distance) and the length of the sheet S (sheet length). The sheet conveying apparatus 100 conveys the sheet S and calculates the sheet conveying distance or the sheet length by using the below-described configuration and method.

The fixing apparatus 50 includes a halogen lamp 57 serving as a heat source. The fixing apparatus 50 also includes a fixing belt (endless belt) 56 and a pressing roller 52 pressed against the fixing belt 56. The fixing apparatus 50 changes a parameter of a fixing condition (e.g., temperatures of the fixing belt 56 and the pressing roller 52, nip width between the fixing belt 56 and the pressing roller 52, rotational speed of pressing roller 52) in accordance with the sheet S. After an image is transferred to the sheet S, the sheet S is conveyed from the secondary transfer apparatus 77 to the fixing apparatus 50 by a conveying belt 41.

When the image forming apparatus 101 receives image data along with a signal instructing to start an image forming process, a driving motor (not illustrated) drives the roller 61 to rotate. Thereby, the rotation of the roller 61 causes the intermediate transfer belt 15 to rotate. At the same time of rotating the intermediate transfer belt 15, each of the developing devices 53 forms a single color image on a corresponding photoconductor drum 71. Then, the single color images formed by the developing devices 53 are sequentially transferred onto the intermediate transfer belt 15 in a superposed manner. Thereby, a composite color image is formed.

Further, one of the sheet feeding rollers 72 of a sheet feeding table 76 is selected and rotated to deliver the sheet S out from one of the sheet feeding cassettes 73. Then, a sheet conveying roller 74 conveys the sheet S until the sheet S contacts a registration roller 75. Then, the registration roller 75 corrects the position for conveying the sheet S. Then, the registration roller 75 conveys the sheet S to the secondary transfer apparatus 77 at a timing that is synchronized with a timing where the composite color image on the intermediate transfer belt 15 reaches the secondary transfer apparatus 77. Thereby, the composite color image on the intermediate transfer belt 15 is transferred to a surface of the sheet S conveyed to the secondary transfer apparatus 77.

After the composite color image is transferred onto the sheet S, the sheet S is conveyed to the fixing apparatus 50 by the conveying belt 41. The transferred image is fixed to the sheet S by applying heat and pressure to the sheet S, so that the transferred image is fused to the sheet S. In a case of performing double-side printing, the sheet S is conveyed to a sheet inverting path 93 by a bifurcating claw 91 and a flip roller 92 after an image is transferred on a front surface side of the sheet S. Then, the sheet S is switched back and conveyed to a double-side conveying path 94 by, for example, a bifurcating claw (not illustrated) and a pair of rollers (not illustrated). Then, a composite color image is formed on a back surface side of the sheet S.

Further, in a case of flipping and discharging the sheet S, the bifurcating claw 91 guides the sheet S to the sheet-inverting path 93. Then, the sheet inverting path 93 flips the sheet S and discharges the sheet S. In a case of printing on a single side of the sheet S and not flipping the sheet S, the bifurcating claw 91 guides the sheet S to a sheet discharging roller 95.

Then, the sheet discharging roller 95 conveys the sheet S to a decurler unit 96. The decurler unit 96 changes the decurling amount depending on the sheet S to be decurled. The decurling amount can be adjusted by changing the pressure exerted from a decurler roller 97. After the decurling amount is adjusted, the decurler roller 97 discharges the sheet S. A purge tray 40 is positioned below a sheet inverting discharge unit.

Instead of using, for example, the registration roller 75, a registration mechanism including a registration gate and a skew correction mechanism may be used to correct a position of the sheet S with respect to the sheet conveying direction or with respect a width direction orthogonal to the sheet conveying direction. In this case, the sheet conveying apparatus 100 controls the timing for conveying the sheet S to a secondary transfer part located between the roller 62 and the secondary transfer roller 14. More specifically, the sheet conveying apparatus 100 controls the speed for conveying the sheet S based on detection results of a sheet detection sensor provided between the registration mechanism and the sheet conveying apparatus 100, so that a timing where a toner image on the intermediate transfer belt 15 reaches the secondary transfer part matches a timing where the sheet S reaches the secondary transfer part.

Although the image forming apparatus 101 of the above-described embodiment has a configuration in which a toner image formed on the intermediate transfer belt 15 is transferred to the sheet S, the image forming apparatus 101 may have a configuration in which single color toner images formed on corresponding photoconductor drums 71 are directly transferred and superposed on the sheet S. Alternatively, the image forming apparatus 101 may be, for example, a monochrome image forming apparatus that forms a single color image. Further, the method for forming an image is not limited to the electrophotographic method. For example, an inkjet method may be used to as the image forming method.

FIG. 2 is a schematic diagram illustrating a configuration of a portion of the image forming apparatus 101 according to an embodiment of the present invention.

As illustrated in FIG. 2, the sheet conveying apparatus 100 conveys the sheet S to the secondary transfer apparatus 77 and measures the sheet length or the sheet conveying distance of the sheet S.

In a case of performing double-side printing, the sheet conveying apparatus 100 measures the sheet length of the sheet S before and after performing an image forming process on one surface (front surface) of the sheet S. Thereby, the sheet conveying apparatus 100 calculates a change of sheet length before and after performing the image forming process. Accordingly, the image forming apparatus 101 corrects the magnification of image data to be printed on another surface (back surface) of the sheet S based on the change of sheet length (in the case of performing double-side printing) calculated by the sheet conveying apparatus 100. Thus, registration accuracy of the image forming apparatus 101 can be improved by correcting the magnification of image data to be formed (printed) on another surface (back surface) of the sheet S.

In the case of performing double-side printing on the sheet S, the sheet S shrinks and changes shape by having heat and pressure applied when passing the fixing apparatus 50 during the process of forming an image on the front surface of the sheet S. Even after the sheet S has passed the fixing apparatus 50, the sheet S continues to change shape as the temperature of the sheet S becomes lower. Therefore, in order to increase the accuracy of correcting the magnification of the image to be formed on the back surface by calculating the sheet length of the sheet S, the sheet length of the sheet S is preferred to be measured immediately before transferring the image on the back surface of the sheet S. Therefore, the sheet conveying apparatus 100 is preferred to be provided at a position that is immediately upstream of the secondary transfer apparatus 77.

<Configuration of Sheet Conveying Apparatus>

Next, a configuration of the sheet conveying apparatus 100 according to an embodiment of the present invention is described with reference to FIGS. 3 and 4. FIG. 3 is a schematic diagram of the sheet conveying apparatus 100 according to an embodiment of the present invention. FIG. 4 is a plan view of the sheet conveying apparatus 100 according to an embodiment of the present invention.

The sheet conveying apparatus 100 includes a conveying unit constituted by, for example, a driving roller 12 and a driven roller 11. The driving roller 12 is driven to rotate by a driving force of a driving unit such as a motor (not illustrated). The driven roller 11 is driven by the driving roller 12 to rotate in conformity to the rotation of the driving roller 12. In a case of conveying the sheet S, the driven roller 11 holds (nips) the sheet S between the driven roller 11 itself and the driving roller 12 and conveys the sheet S.

As illustrated in FIG. 4, a width Wr of the driven roller 11 (length of the driven roller 11 in a sheet width direction orthogonal to the sheet conveying direction of the sheet S) is less than a minimum width Ws of the sheet S used for the sheet conveying apparatus 100. In the case of conveying the sheet S, the driven roller 11 does not contact the driving roller 12. Therefore, the driven roller 11 is rotated solely by the friction generated between the driven roller 11 and the sheet S. Accordingly, in the case of conveying the sheet S, the driven roller 11 can calculate the sheet conveying distance or the sheet length of the sheet S without being affected of the driving roller 12. Thereby, the accuracy for calculating the sheet conveying distance or the sheet length of the sheet S can be increased.

A rotary encoder 21 is provided on a rotational axle of the driven roller 11 of the sheet conveying apparatus 100. Details of the configuration of the rotary encoder 21 are described below.

A sensor (detection unit) 3 is provided in the vicinity of the driving roller 12 and the driven roller 11 on their downstream side with respect to the conveying direction of the sheet S. Further, a sensor (detection unit) 4 is provided in the vicinity of the driving roller 12 and driven roller 11 on their upstream side with respect to the conveying direction of the sheet S. The sensors 3, 4 detect the passing of an end part of a conveyed sheet S. Transmission type sensors or reflection type sensors that can detect, for example, an end part of a sheet S may be used as the sensors 3, 4. In this embodiment, reflection type sensors are used as the sensors 3, 4.

The sensor 3, which is provided in the vicinity of the driving roller 12 and the driven roller 11 on their downstream side with respect to the conveying direction of the sheet S, is a start trigger sensor (downstream detection unit) that detects a front end part of the sheet S. The sensor 4, which is provided in the vicinity of the driving roller 12 and driven roller 11 on their upstream side with respect to the conveying direction of the sheet S, is a stop trigger sensor (upstream detection unit) that detects a rear end part of the sheet S.

As illustrated in FIG. 4, the sensors 3, 4 are arranged substantially at the same position in a width direction that is orthogonal to the conveying direction of the sheet S. By arranging the sensors 3, 4 in this manner, skewing with respect to the sheet conveying direction (conveyed position of the sheet S) has a minimal effect in measuring the sheet conveying distance of the sheet S. Thereby, the sheet conveying distance of the sheet S can be measured more accurately.

In this embodiment, the sensors 3, 4 are arranged at a center position in the width direction that is orthogonal to the conveying direction of the sheet S. However, the sensors 3, 4 may be arranged in a position deviating from the center position of the width direction as long as the sensors 3, 4 are positioned within an area through which the sheet S passes.

Distance A illustrated in FIGS. 3 and 4 represents the distance of the sensor (start trigger sensor) 3 in the sheet conveying path with respect to the driven roller 11 and the driving roller 12. Further, distance B illustrated in FIGS. 3 and 4 represents the distance of the sensor (stop trigger sensor) 4 in the sheet conveying path with respect to the driven roller 11 and the driving roller 12. The distances A and B are preferred to be as short as possible because the below-described pulse count range increases as the distances A and B become longer.

The driving roller 12 is rotated by a driving unit (not illustrated) in an arrow direction illustrated in FIG. 3. The driven roller 11 is rotated in conformity to the rotation of the driving roller 12 in a case where the sheet S is not being conveyed. In a case where the sheet S is being conveyed, the driven roller 11 is rotated in conformity to the conveyed sheet S. When the driven roller 11 is rotated, a pulse(s) signal is output from the rotary encoder 21 provided on the rotational axle of the driven roller 11.

The below-described pulse counting unit 23 (see FIG. 7) connected to the rotary encoder 21 counts the pulse signals output from the rotary encoder 21. The count results of the pulse counting unit 23 (see FIG. 7) are used to calculate the sheet conveying distance or the sheet length of the sheet S.

(Configuration of Rotary Encoder)

As illustrated in FIGS. 3 and 4, the rotary encoder 21 includes, for example, an encoder disk 18 having slits 19 provided at its periphery, and sensors 20 a, 20 b that detect the slits 19 of the encoder disk 18 and output pulse signals (detection signals). The pulse signals output from the sensors 20 a, 20 b are counted by the pulse counting unit 23 (see FIG. 7) and used for calculating the sheet conveying distance or the sheet length of the sheet S.

As illustrated in FIG. 3, multiple slits 19 are provided at substantially equal intervals in a peripheral direction of the encoder disk 18. The sensors 20 a, 20 b, which detect the slits 19 and output pulse signals, are provided substantially opposite to each other where the rotational axle of the encoder disk 18 is positioned at the center between the sensor 20 a and the sensor 20 b.

FIG. 5 is a schematic diagram illustrating the number of pulse counts of the pulse signals output from the sensors 20 a, 20 b of the rotary encoder 21 during constant rotation of the driven roller 11 according to an embodiment of the present invention.

As illustrated in FIG. 5, in a case where the encoder disk 18 is eccentric with respect to the rotational axle of the driven roller 11, the number of pulse counts of the pulse signals output from the sensor 20 a and the number of pulse counts of the pulse signals output from the sensor 20 b cyclically (periodically) change, respectively. Because the sensors 20 a, 20 b are provided substantially opposite to each other where the rotational axle of the encoder disk 18 is positioned at the center between the sensor 20 a and the sensor 20 b, the timing of the cyclic change of the number of pulse counts output from the sensor 20 a and the timing of the cyclic change of the number of pulse counts output from the sensor 20 b are half a cycle deviated from each other.

Owing to the deviation, the influence of the eccentricity of the encoder disk 18 can be canceled out (set off). Thus, as shown in FIG. 5, the average value of the number of pulse counts of the pulse signals output from the sensors 20 a, 20 b is illustrated as a straight line that is proportional to time t. Similarly, because the influence of the eccentricity of the encoder disk 18 can be canceled, the total value (not illustrated) of the number of pulse counts of the pulse signals output from the sensors 20 a, 20 b is illustrated as a straight line that is proportional to time t.

Therefore, by using the average value and the total value of the number of pulse counts of the pulse signals output from the sensors 20 a, 20 b, varying of the number of pulse counts due to inconsistent measurement timing can be prevented. Because the above-described configuration of the sheet conveying apparatus 100 prevents the varying of the number of pulse counts, the sheet conveying apparatus 100 can calculate the sheet conveying distance or the sheet length of the sheet S with high accuracy.

Further, the multiple slits 19 are preferred to be provided in the encoder disk 18 in a manner that causes the phases of the pulse signals output from the sensor 20 a to be different from the phases of the pulse signals output from the sensor 20 b as illustrated in FIG. 6. In this embodiment, the multiple slits 19 are provided in the encoder disk 18, so that the phases of the pulse signals output from the sensors 20 a and 20 b are deviated 90°.

Owing to the arrangement of the slits 19 in the encoder disk 18, a pulse signal output from the sensor 20 b is output at a timing deviated half a cycle (½) with respect to a timing of outputting a pulse signal from the sensor 20 a in a case where the encoder disk 18 is rotated in correspondence with the driven roller 11. It is to be noted that the phase difference between the pulse signal output from the sensor 20 a and the pulse signal output from the sensor 20 b is not limited to the phase difference described in this embodiment. That is, the phase difference between the pulse signal output from the sensor 20 a and the pulse signal output from the sensor 20 b may be arbitrarily set.

In a case of counting a pulse signal output from a single sensor, the maximum delay of detecting a pulse signal with respect to the timing of starting to count pulse signals (counting start timing) is equivalent to a single cycle t of a pulse signal. However, according to this embodiment, by using the sensors 20 a and 20 b arranged in the above-described manner, the maximum delay of detecting a pulse signal with respect to the timing of starting to count pulse signals (counting start timing) is shortened to half of the cycle t of a pulse signal. Further, owing to the shortened interval between the pulse signal output from the sensor 20 a and the pulse signal output from the sensor 20 b, more pulse signals can be counted at the timing of stopping to count pulse signals (counting stop timing). Therefore, because counting of pulse signals can be started with high sensitivity throughout the counting start timing to the counting stop timing, the sheet conveying distance or the sheet length of the sheet S can be calculated with higher accuracy.

Although a similar effect may be attained by increasing the number of slits 19 provided in the encoder disk 18, it may be difficult to narrow the intervals between the slits 19 due to, for example, constraints in manufacture technology and constraints of resolution of the sensors 20 a, 20 b. Further, increasing the number of slits 19 by increasing the outer diameter of the encoder disk 18 results in increasing the size of the sheet conveying apparatus. However, the resolution of the output of the sensors 20 a, 20 b can be improved without increasing the number of slits 19 or increasing the size of the sheet conveying apparatus 100. Thereby, the sheet conveying distance or the sheet length of the sheet S can be calculated with high accuracy.

Although the two sensors 20 a, 20 b are provided in the above-described configuration of rotary encoder 21, the number of sensors provided in the rotary encoder 21 is not limited to two. For example, two sets of sensors may be provided substantially opposite to each other where the rotational axis of the encoder disk 18 is positioned at the center between the first set of sensors and the second set of sensors.

The driven roller 11 attached to the rotary encoder 21 is preferred to have a small diameter. This is because the number of rotations of the driven roller 11 corresponding to the conveying of the sheet S increases as the diameter of the driven roller 11 becomes smaller. Thus, as the number of rotations of the driven roller 11 increases more pulse signals can be counted. As a result, the sheet conveying distance or the sheet length of the sheet S can be calculated with high accuracy.

The driven roller 11 attached to the rotary encoder 21 is preferred to be formed of metal for ensuring precision of axial runout. By controlling axial runout of the rotational axle of the driven roller 11, the below-described calculation of sheet conveying distance or sheet length of the sheet S can be performed with high accuracy.

<Functional Configuration of Image Forming Apparatus>

FIG. 7 is a block diagram illustrating a functional configuration of the image forming apparatus 101 according to an embodiment of the present invention.

As illustrated in FIG. 7, the image forming apparatus 101 includes, for example, the start trigger sensor 3, the stop trigger sensor 4, a sheet shape calculating unit 20, the rotary encoder 21, the pulse counting unit 23, a conveyance distance calculating unit 25, and an image data correcting unit 27. The functions of sheet shape calculating unit 20, the pulse counting unit 23, the conveyance distance calculating unit 25, and the data correcting unit 27 are implemented by cooperatively operating with hardware such as a RAM (Random Access Memory) 28, a CPU (Central Processing Unit) 29, and a ROM (Read Only Memory) 29 having a program stored therein.

The pulse counting unit 23 counts the pulse signals output from the sensors 20 a, 20 b in accordance with the rotation of the encoder disk 18 of the rotary encoder 21 attached to the driven roller 11.

The conveyance distance calculating unit 25 calculates the sheet conveying distance or the sheet length of the sheet S based on the detection results of the start trigger sensor 3 and the stop trigger sensor 4 and the number of pulse counts of the pulse counting unit 23.

The image data correcting unit 27 corrects image data formed on a back surface of the sheet S by the image forming apparatus 101 based on the sheet conveying distance or the sheet length calculated by the conveyance distance calculating unit 25.

By correcting the image data with the image data correcting unit 27, the image forming apparatus 101 can print images with high registration accuracy in a case of performing double-side printing on the sheet S.

<Calculation of Sheet Conveying Distance and Sheet Length>

Next, a method for calculating the sheet conveying distance and the sheet length of the sheet S with the image forming apparatus 101 is described.

FIG. 8 is a schematic diagram illustrating outputs of the start trigger sensor 3, the stop trigger sensor 4, the sensor 20 a, and the sensor 20 b according to an embodiment of the present invention.

As described above, the sensors 20 a, 20 b of the rotary encoder 21 provided on the rotational axle of the driven roller 11 output pulse signals when the driven roller 11 is rotated.

After conveying of the sheet S is started in the example illustrated in FIG. 8, the stop trigger sensor 4 detects the passing of the front end part of the sheet S at time t1. Then, the start trigger sensor 3 detects the passing of the front end part of the sheet S at time t2.

Then, the stop trigger sensor 4 detects the passing of the rear end part of the sheet S at time t3. Then, the start trigger sensor 3 detects the passing of the rear end part of the sheet S at time t4.

The pulse counting unit 23 counts the pulse signals that are output from the sensors 20 a, 20 b during a pulse counting period starting from the detection of the passing of the front end part of the sheet S by the start trigger sensor 3 (i.e. time t2) and ending upon the detection of the passing of the rear end part of the sheet S by the stop trigger sensor 4 (i.e. time t3).

The following Formula (1) is used to calculate a distance (sheet conveying distance) Ld that the sheet S is conveyed between time t2 and time t3.

Ld=(n/N)×2πr  <Formula (1)>

In Formula (1), “r” represents a radius of the driven roller 11 having the rotary encoder 21 attached thereto [mm]; “N” represents the number of encoder pulses (total number of pulse signals output from the sensors 20 a, 20 b) per a single rotation of the driven roller 11 [/r]; and “n” represents the total number of pulse signals output from the sensors 20 a, 20 b during the pulse counting period.

Generally, the sheet conveying speed changes depending on, for example, the precision of the outer shapes of the rollers (particularly, driving roller 12) that convey the sheet S, mechanical precision such as precision of axial (core) runout, rotation precision of a motor or the like, and precision of drive mechanisms such as gears and belts. Further, the sheet conveying speed may also change due to, for example, a slipping phenomenon between the driving roller 12 and the sheet S, or a deflection phenomenon caused by difference of conveying strengths of a sheet conveying unit on an upstream side and a downstream side or difference of sheet conveying speeds on an upstream side and a downstream side. However, the number of pulse counts of the pulse signals output from the sensors 20 a, 20 b of the rotary encoder 21 increases in proportion with the sheet conveying distance regardless of the sheet conveying speed.

Therefore, regardless of factors such as sheet conveying speed, the conveyance distance calculating unit 25 can accurately calculate the distance Ld in which the sheet S is conveyed by the sheet conveying unit (driven roller 11, driving roller 12) by using the above-described Formula (1).

It is to be noted that the sheet conveying distance Ld can also be calculated by using the average value of the number of pulse counts of the pulse signals output from the sensors 20 a, 20 b. In this case, “n” of Formula (1) is assumed to be the average value of the number of pulse counts, and “N” of Formula (1) is assumed to be the number of slits 19 provided in the encoder disk 18.

Further, the conveyance distance calculating unit 25 can also calculate a relative ratio (proportion) such as a ratio between the pages of sheet S or a ratio between a front surface and a back surface of the sheet S.

The conveyance distance calculating unit 25 can also calculate a shrinkage ratio R by using the following Formula (2) based on a relative ratio between the sheet conveying distances calculated before and after forming an image(s) with the image forming apparatus 101.

R=[(n2/N)×2πr]/[(n1/N)×2πr]  <Formula (2)>

In Formula (2), “n1” represents the total number of pulses counted during the conveying of the sheet S before thermally fixing an image onto the sheet S, and “n2” represents the total number of pulses counted during the conveying of the sheet S after thermally fixing the image to the sheet S.

Examples using the above-described formulas are described below.

In this embodiment, the sheet conveying distance L1 of the sheet S can be expressed as follows in a case where the sheet S is a A3 size paper, N=2800 [/r], r=9 [mm], and n1 (total number of pulse signals counted when vertically conveying the sheet S)=18816.

L1=(18816/2800)×2π×9=380.00 mm

Further, in a case where n2 (total number of pulse signals counted after thermal fixing an image to the sheet S)=18759, the sheet conveying distance L2 is expressed as follows.

L2=(18759/2800)×2π×9=378.86 mm

A difference ΔL between the sheet conveying distance of the sheet S for the front side and the sheet conveying distance of the sheet S for the back side is expressed as follows.

ΔL=380.00−378.86=1.14 mm

Based on the calculation results of the sheet conveying distance of the front and back sides of the sheet S, a shrinkage rate R of the sheet S (i.e. relative ratio between length of the front side of the sheet S and length of the back side of the sheet S) is expressed as follows.

R=378.86/380.00=99.70%

In this case, the length of the sheet S is reduced (shrunk) approximately 1 mm after performing thermal fixing on the sheet S. Therefore, a misalignment of approximately 1 mm would occur for an image printed on the front and back surfaces of the sheet S if the image is printed with the same length on the front and back surfaces of the sheet S. In order to prevent such misalignment, the image data correcting unit 27 corrects the length of the image to be printed on the back surface of the sheet S based on the shrinkage rate R. Thereby, the accuracy of front-back registration of the image forming apparatus 101 can be improved.

In the above-described embodiment, the shrinkage rate R is obtained by calculating the sheet conveying distances L1, L2 of the sheet S conveyed by the sheet conveying unit (driven roller 11, driving roller 12) before and after performing a thermal fixing process on the sheet S. Alternatively, a shrinkage rate calculating unit may be provided in the image forming apparatus 101 to calculate the shrinkage rate R based on, for example, a ratio of the number of pulses counted during the conveying of the sheet S before and after performing a thermal fixing process on the sheet. Thus, for example, in the above-described case where n1=18816 and n2=18759, the shrinkage rate R can be obtained as follows.

R=n2/n1=18759/18816=99.70%

It is to be noted that, as shown in the following Formula (3), a sheet length Lp can be obtained by adding a distance “a” between the start trigger sensor 3 and the stop trigger sensor 4 to the sheet conveying distance Ld obtained by Formula (1).

Lp=(n/N)×2πr+a  <Formula (3)>

Accordingly, the conveyance distance calculating unit 25 can calculate the sheet length Lp by using the above-described Formula (3) where the distance “a” between the start trigger sensor 3 and the stop trigger sensor 4 is added to the sheet conveying distance Ld obtained by Formula (1).

The conveyance distance calculating unit 25 can also calculate a shrinkage ratio R by using the following Formula (4) based on a relative ratio between the sheet lengths calculated before and after forming an image(s) with an electrophotographic method and thermally fixing the image(s).

R=[(n2/N)×2πr+a]/[(n1/N)×2πr+a]  <Formula (4)>

With the image forming apparatus 101 according to the above-described embodiment, the conveyance distance calculating unit 25 can calculate the sheet conveying distance or the sheet length of the sheet S with high accuracy. Further, according to necessity, the conveyance distance calculating unit 25 can also calculate the shrinkage ratio R before and after forming an image on the sheet S.

<Method for Correcting Image Data>

Next, the processes of correcting magnification of an image based on a shape of the sheet S calculated by a sheet shape calculating unit 20. In this embodiment, the sheet shape calculating unit 20 calculates the shape of the sheet S at an area positioned immediately before the secondary transfer roller 14 (immediately upstream of the secondary transfer roller 14 in the conveying direction of the sheet S). Accordingly, correction of exposure size or exposure timing based on the calculated shape of the sheet S is not performed for an image to be printed on the sheet S on which the shape calculation is performed. Instead, the correction of exposure size or exposure timing is performed for an image to be printed on a subsequent (latter) sheet S.

In this embodiment, the exposing apparatus 55 of the image forming apparatus 101 includes, for example, a data buffer part for buffering input data stored in a memory or the like, an image data generating part for generating image data to be used for forming an image, an image magnification correcting part for correcting magnification of an image in a sheet conveying direction based on sheet size data, a clock generating part for generating a write clock, and an illuminating device for radiating light to the photoconductor drum 71 and forming an image.

The data buffer part buffers input image data from a host apparatus (e.g., controller) by using transfer clocks.

The image data generating part generates image data based on write clocks from the clock generating part and pixel insertion/extraction data from the image magnification correcting part. Further, drive data output from the image data generating part is used to perform ON/OFF control of the illuminating device assuming that the length of a single cycle of the write clock is a single pixel of an image to be formed.

The image magnification correcting part generates an image magnification switch signal for switching magnification of an image based on a sheet shape calculated by the sheet shape calculating unit 20 of the image forming apparatus 100.

The clock generating part operates with a frequency that is several times higher than that of the write clock for changing clock cycles and performing image correction such as the known method of pulse width modulation. The clock generating part basically generates write clocks at a frequency corresponding to the rate of the apparatus to be used.

The illuminating device may include one or more of, for example, a semiconductor laser, a semiconductor laser array, and a plane emission laser to form an electrostatic latent image by radiating light to the photoconductor drum 71 according to drive data.

Hence, according to the above-described embodiments of the present invention, the sensors 20 a, 20 b, which are provided substantially opposite to each other where a rotational axle of encoder disk 18 is positioned at a center between the sensors 20 a, 20 b, can detect the slits 19 and output pulse signals. Even in a case where the encoder disk 18 is eccentric with respect to the rotational axle of the driven roller 11, the total value or the average value of the pulse signals output from the sensors 20 a, 20 b is proportional to the rotation amount of the driven roller 11. Accordingly, the influence of the eccentricity of the encoder disk 18 can be canceled out (offset), and the sheet conveying distance or the sheet length of the sheet can be accurately calculated in accordance with the total value or the average value of the pulse signals output from the sensors 20 a, 20 b.

Further, the image forming apparatus 101 according to the above-described embodiment of the present invention can correct magnification of the image data to be printed on a back surface (other surface) of the sheet S based on the changes of sheet length that is accurately calculated with the sheet conveying apparatus 100 during double-side printing. Thereby, the front/back registration accuracy of the image forming apparatus 101 can be improved.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

The present application is based on and claims the benefit of priority of Japanese Priority Application No. 2013-087304 filed on Apr. 18, 2013, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A sheet conveying apparatus comprising: a detecting unit configured to detect a sheet; a conveying unit including a driving roller and a driven roller that rotates in conformity with the driving roller and configured to convey the sheet by holding the sheet between the driving roller and the driven roller; a disk configured to rotate in correspondence with the driven roller and including a plurality of slits arranged in a periphery thereof; first and second sensors configured to detect the plurality of slits and output detection signals, the first and second sensors being provided substantially opposite to each other where a rotational axle of the disk is positioned at a center between the first and second sensors; and a conveyance distance calculating unit configured to calculate a sheet conveying distance based on a detection result of the detecting unit and the detection signals.
 2. The sheet conveying apparatus as claimed in claim 1, wherein the plurality of slits are arranged to cause a phase of the detections signals output from the first sensor to be different from a phase of the detection signals output from the second sensor.
 3. The sheet conveying apparatus as claimed in claim 1, wherein the detecting unit includes an upstream detecting unit provided on an upstream side of the conveying unit with respect to a sheet conveying direction for detecting the sheet, and a downstream detecting unit provided on an downstream side of the conveying unit with respect to the sheet conveying direction for detecting the sheet.
 4. The sheet conveying apparatus as claimed in claim 3, wherein the conveyance distance calculating unit is configured to calculate the sheet conveying distance based on the detection signals during a period between detection of the sheet by the downstream detecting unit and detection of the sheet by the upstream detecting unit.
 5. The sheet conveying apparatus as claimed in claim 4, wherein the conveyance distance calculating unit is further configured to calculate a length of the sheet in the sheet conveying direction by adding the sheet conveying distance to a distance between the upstream detecting unit and the downstream detecting unit.
 6. An image forming apparatus comprising: the sheet conveying apparatus as claimed in claim
 1. 7. The image forming apparatus as claimed in claim 6, further comprising: a transfer unit configured to transfer an image to the sheet; wherein the sheet conveying apparatus is provided directly above the transfer unit.
 8. The image forming apparatus as claimed in claim 6, further comprising: an image forming unit configured to form the image to be transferred based on image data; and a correcting unit configured to correct the image data based on the sheet conveying distance calculated by the conveyance distance calculating unit. 